The preparation of brominated pentaerythritols has been carried out by reacting fuming hydrobromic acid with pentaerythritol at temperatures of 120.degree. to 160.degree.C. for periods of 8-15 hours and by reacting hydrogen bromide with pentaerythritol in glacial acetic acid; see Berlow, Barth and Snow, The Pentaerythritols, ACS Monograph No. 136, pp. 99-100. In the reactions with hydrobromic acid, long reaction times and high temperatures are required. Reactions carried out in glacial acetic acid form large amounts of undesirable acetate by-product; moreover, in the prior art processes large amounts of tar are formed, and where such formation is obviated, it is obtained at the expense of having to use extremely high pressures.
Thus, Conia et al., Bull. Soc. Chim. France, 1961, p. 1803, discloses a process whereby pentaerythritol and HBr are reacted using acetic acid in large amounts as a solvent. This process produces an unknown mixture of brominated neopentyl acetates which have to be hydrolyzed back to the alcohols, stripped and distilled. The yield or products are not revealed. The melting point of this unknown product is 90.degree.-95.degree.C. which is neither that of tribromoneopentyl alcohol (72.degree.C.) nor dibromoneopentyl glycol (110.degree.C.).
Bincer et al., Ber. 61 B (1928), p. 542, discloses the reaction of pentaerythritol with aqueous HBr using no catalyst at 140.degree.C. for 20 hours. Using elevated pressures, they obtain a 44% tribromoneopentyl alcohol yield which is exemplified by a dark color.
Zelinsky, Ber. 46 (1913), pp. 163-164, also reacts pentaerythritol with aqueous HBr in the absence of a catalyst and obtains at 44% yield. He uses a sealed system under pressure and obtains a "very dark" product.
In the somewhat related processes whereby pentaerythritol chlorohydrins are formed, processes are disclosed where either large amounts of acetate by-products are formed necessitating saponification, unduly and undesirable high pressures are used in the process, and/or the products are replete with tar formation, necessitating distillation. In the bromination of pentaerythritols, saponification of the reaction product to reduce acetate content is not feasible because the rate of decomposition of the brominated pentaepythritol is greater than the rate of saponification. Consequently, the oxetane is formed, plus irreversible lachrymatory (2-bromomethyl-3-bromo-propene) as well as other undesirable by-products.
Thus, British Pat. No. 764,664 discloses a process for preparing pentaerythritol trichlorohydrin by reacting pentaerythritol with at least 3 moles and preferably 5 to 10 moles of hydrogen chloride at extremely high pressures and temperatures between 150.degree. and 200.degree.C. in the presence of at least 10 mole percent of an aliphatic monocarboxylic acid or ester of same per each mole of pentaerythritol. An inert organic solvent may be used. All the ingredients are combined at the outset of the reaction and reacted in a silver-plated autoclave. Saponification is used in all cases to transform the monoacetate formed into free trichlorohydrin. At the lower range of acetic acid used, the yield is only 63%. This process has the disadvantage that it must be reacted at elevated temperatures and pressures. As indicated in U.S. Pat. No. 3,217,045, the British Pat. No. 764,664 process has little to recommend it. In Column 1, lines 30-39 of the cited U.S. patent, reference is made to the British patent's German equivalent patent specification (No. 955,233). It is there stated that "this process does not proceed in the manner specified unless elevated pressures are applied. The process is difficult to carry out on a commercial scale on account of the aqueous hydrochloric acid which forms during the reaction . . ." Likewise, Japanese Pat. No. 39-27230 points out that the process of British Pat. No. 764,664 requires the use of pressure equipment and presents problems in operation and recovery of the product.
Japanese Pat. No. 39-27230 discloses the manufacture of pentaerythritol trichlorohydrin through a direct chlorination of pentaerythritol with HCl gas with a small amount of acetic anhydride. This process teaches the use of 3 to 50 mole equivalents of acetic anhydride at 170.degree.-200.degree.C. to react HCl gas with pentaerythritol. It obviates the use of high pressures in the reaction; however, because they remove the water that is formed during the reaction continuously throughout the reaction by passing large excesses of HCl to sweep it out, the Japanese obtain large amounts of tar formation at low levels of acetic anhydride. Consequently they teach that the preferred range of acetic anhydride is 25 to 50 mole equivalents. This corresponds to a 50 to 100 mole percent of carboxylic radical per mole equivalent of pentaerythriol. The Japanese patent indicates that at low levels of acetic anhydride, yield is drastically reduced and etherization (tar formation) becomes more prominent than chlorination. As will be shown below, at the levels of acetic acid used in the instant invention, the Japanese process produces about 1 pound of tar per 1 pound of usable product. Finally, the Japanese state that acetic acid cannot be used as a catalyst.
These significant drawbacks to the preparation of brominated pentaerythritols have led to an extensive search for methods of avoiding these problems.